Your ranking consists of 23 tournaments. The 4 Grand Slams + 10 Masters 1000s + 10 of your other best results (from the 250s and 500s). If you qualify for the WTF at the end of the year that adds as your 24th tournament.

Since the Grand Slams and 1000 tournaments are mandatory, missing any of them results in a score of "0"

Only your best result of the week is counted each week towards your ranking

The Race differs from the Rankings because Rankings only count 24 tournaments, whereas the Race includes your results from every week of the calendar year.